1. Field of the Invention
The present invention relates to an automatic focusing apparatus for a camera or the like.
2. Related Background Art
Known so-called pursuit or follow-up drive technique, a defocus amount of a photographing lens is repetitively detected, and it is determined on the basis of the present and previous defocus amounts whether or not an object to be photographed is moving. If it is determined that the object is moving, a correction amount for correcting the position of the photographing lens with respect to the moving object is calculated on the basis of the present and previous defocus amounts, and a drive amount of the photographing lens is calculated on the basis of a sum of the defocus amounts and the correction amount, thereby driving the photographing lens with respect to the moving object without being delayed. For example, this technique is disclosed in Japanese Patent Laid-Open (Kokai) No. 60-214325 filed by the present applicant.
In the conventional technique described above, when the drive direction of the photographing lens is reversed, the following problem is posed.
Since an initial drive operation of the photographing lens immediately after reversal includes backlash of a drive system, a difference between an estimated drive amount and an actual drive amount is produced. Thus, although the photographing lens is moved by the estimated drive amount in discrimination of the moving object after the initial drive operation, a defocus amount corresponding to the difference is detected. In the conventional technique, it is erroneously determined that this defocus amount is caused by movement of the object. Thus, the pursuit drive operation for the moving object is performed, and hence, the photographing lens overruns from an actual in-focus position.
In particular, the backlash amount and the defocus amount are amounts on the same order near the in-focus position, and the photographing lens engages in hunting around the in-focus position due to the pursuit drive operation and cannot be easily converged to the in-focus position.
When the drive direction is reversed, backlash data stored in a ROM incorporated in the lens is read out to correct the backlash of the photographing lens, and the above drawback can be alleviated to some extent.
However, the backlash amount varies even in identical types of lenses, and changes over time. Thus, the conventional drawback cannot be completely eliminated.